This invention relates to a system for stopping a sprindle at a predetermined rotational position, and more particularly to a system for stopping a specified point on a sprindle at either of two predetermined rotational positions.
Some machine tools which are known in the art have an automatic tool changing function which allows machining to be performed while a variety of tools mounted on the machine are interchanged automatically. The tool changing operation proceeds as follows. First, a magazine holding a number of tools is revolved to bring a vacant tool holding portion of the magazine into position directly above a spindle mechanism. The spindle mechanism, which is grasping an old tool to be exchanged for a new one, is then projected forwardly, after which the magazine positioned above the spindle mechanism is lowered to permit the old tool to be received and grasped by the vacant tool holding portion of the magazine. The spindle mechanism is then retracted so that the old tool separates from the spindle, thereby transferring the old tool to the magazine. Next, the magazine is revolved to bring a desired new tool into position in front of the sprindle, and the spindle mechanism is projected forwardly to allow the spindle to receive and to grasp the new tool. Finally the magazine is retracted away from the spindle to complete the tool change operation.
It is required, in the tool change mechanism of the foregoing type, that a prescribed part of the spindle, such as a key member, be stopped accurately at the correct rotational position in order to permit the fitting portions of the spindle and tool to mate with each other smoothly. More specifically, a key is mounted on the spindle and a keyway is formed in the tool in order to mate with the key. The smooth mating of the spindle and tool requires that the spindle be positioned and stopped to provide the correct alignment of key and keyway. Meeting the above requirement necessitates a high spindle positioning accuracy of from .+-.0.1 to .+-.0.2 degrees in terms of the angle of rotation of the spindle.
The conventional automatic tool change mechanisms are provided with photoelectric detectors or with limit switch mechanisms which detect the rotational position of the spindle key in order to facilitate the smooth mating of the spindle and tools. The arrangement is such that the spindle is brought to a stop at the prescribed position by the application of a mechanical brake which is actuated in response to a signal from the key position detecting means.
The foregoing apparatus employs a stopping mechanism that experiences wear with a long period of use since the mechanism relies upon a mechanical pin or brake or the like. Such wear, particularly of a brake shoe or pin, makes it progressively more difficult to stop the spindle at the predetermined position, and the result is that the automatic changing of tools cannot proceed smoothly.
Accordingly, there is a need for a control system which is capable of stopping a spindle at a predetermined rotational position with a high degree of accuracy by purely electrical means, i.e., without relying upon a mechanical pin or mechanical brake mechanism to stop the spindle, in performing an automatic tool change operation.
On the other hand, numerically controlled machine tools are being applied increasingly to the boring of such workpieces as automobile engine boxes. Such boring work requires the use of thicker boring bars or cutters in order to prevent chatter by increasing rigidity. There are cases, however, where the use of thicker diametered boring bars cannot be avoided, as in the case of boring a hole whose dimensions are such that the thicker boring bars cannot be inserted. This point will be elaborated on in the following description of a boring machine tool.
FIG. 1 is an illustrative view of a boring machine tool in simplified form, including a headstock 101, a boring bar 102, and a cutting tool 103. A workpiece 104, which is carried on a table 105, has holes 104a, 104a' for receiving the boring bar 102, and a hollow portion 104b. In a boring machine tool of this type, the cutting tool 103 is inserted into the hollow portion 104b of the workpiece 104 through either of the insertion holes 104a or 104a', after which the workpiece 104 is moved relative to the cutting tool 103 to permit the cutting tool 103 to bore the workpiece 104 in a prescribed manner. The holes 104a, 104a', since they are eventually to be covered, are comparatively small in diameter. It is therefore necessary to select a small diameter boring bar 102, as shown in FIG. 2, in order to prevent the cutting tool 103 from contacting the periphery of the hole 104a when the boring bar 102 is inserted into and withdrawn from the interior of the workpiece 104. The use of a small diameter boring bar results in chatter during machining, so that a high degree of accuracy cannot be achieved.
Proposed methods of solving the above problem are shown in FIGS. 3 and 4, wherein arrangements are adopted that permit the utilization of a boring bar 102 having a larger diameter. In accordance with the method of FIG. 3, the center of the boring bar 102 is offset from the center of the hole 104a along the Y-axis when the boring bar is inserted and withdrawn, the cutting tool 103 being positioned so as to coincide with the Y-axis. It should be noted, however, that the choice of the Y-axis here is illustrative only. In accordance with the method of FIG. 4, a cutting tool insertion notch 104c is formed in the workpiece 104 so as to communicate with the insertion hole 104a, and the cutting tool 103 is positioned so as to coincide with the notch 104c when the boring bar is inserted and withdrawn. Both methods depicted in FIGS. 3 and 4 reduce chatter by allowing the use of larger diameter boring bar 102. When the cutting tool 103 is inserted into the workpiece and when its rotation is stopped after a machining operation, both of the above methods require that the spindle of the machine tool be stopped accurately at a predetermined rotational position which is the positive Y-axis in the arrangement of FIG. 3 and the position of the cutting tool insertion slot 104c in the arrangement of FIG. 4. In other words, in order to eliminate chatter and effect a rigid machining operation by employing a larger diameter boring bar, a control system is required through which the spindle mounting the boring bar can be stopped at a predetermined rotational position.
The foregoing control of spindle stopping position is thus required for both automatic tool change and machining operations; and in general, since the positions at which the spindle is stopped differ in each case, there is an additional requirement that the spindle be stoppable at either of two rotational positions.